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Nacreous Clouds over Norway. Imaged by Marius Sollerud at Hvervenbukta, 5 miles south of Oslo during the great display of January 20, '08. ©Marius Sollerud, shown with permission.

The season for nacreous clouds is back. 9-16 miles (15 - 25 km) high in the stratosphere, they require the very low temperatures ( -85°C) found in winter. They are helped by strong tropospheric winds over mountains that generate the gravity waves to loft up water vapour to those heights. Look for their intensely bright iridescence shortly before dawn or after sunset. They are found most frequently over Scandinavia, Alaska, and Northern Canada but can be seen more rarely further south.

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